Questions for '71 Cutlass Supreme differential

[Omicron]

How does one tell what gear ratio, and if it's positraction (besides just one tire spinning ), on a factory rear end of my '71 CS?

Secondly, if it's not posi and I change it, will it be strong enough to handle 650 BHP? If not, what's a good source for stronger internals?

Thanks in advance!

[joe_padavano]

Originally Posted by Omicron

How does one tell what gear ratio, and if it's positraction (besides just one tire spinning ), on a factory rear end of my '71 CS?

Secondly, if it's not posi and I change it, will it be strong enough to handle 650 BHP? If not, what's a good source for stronger internals?

Thanks in advance!

Jack up the rear of the car. With the trans in park, try spinning one rear tire by hand. If the other tire turns in the opposite direction, it's an open diff. If you can't turn the tire, it's a posi.

[Omicron]

Originally Posted by joe_padavano

Jack up the rear of the car. With the trans in park, try spinning one rear tire by hand. If the other tire turns in the opposite direction, it's an open diff. If you can't turn the tire, it's a posi.

Man Joe, you are just a wealth of knowledge! Thanks AGAIN! Any easy way to tell what gear ratio the car has?

[Omicron]

Actually, just found this:

Originally Posted by smcurro

An easier way to figure out the ratio is if the axle is a posi turn the axle one full rotation and count the rotation of the pinion. If the differential is open then fix one side of the axle in place and turn the other side two full rotations again counting the rotation of the pinion. If the pinion turns three and a half rotations, your gears are 3.50 to 1. This is much easier than taking things apart.

Originally Posted by joe_padavano

Unfortunately, it can sometimes be difficult to tell the difference between 3.23 turns, 3.31 turns. and 3.42 turns. I turn the pinion until the axle shaft turns ten times. It's much easier to tell the difference between 32.3, 33.1, and 34.2, for example.

As for available ratios, it could have been anything between 2.73:1 and 5.00:1.

So I think I'm ok.

[J-(Chicago)]

With the pumpkin cover off & transmission in neutral....Count as you spin the wheel by hand. Mark one tooth with white out or chalk so you know where you started.
Axles connected by gears are always synchronized exactly with one another.
They make it possible to determine exact gear ratios - you just count the number of teeth in the two gears and divide. So if one gear has 60 teeth and another has 20, the gear ratio when these two gears are connected together is 3:1.
They make it so that slight imperfections in the actual diameter and circumference of two gears don't matter. The gear ratio is controlled by the number of teeth even if the diameters are a bit off.

[88 coupe]

Counting revolutions of the tires vs driveshaft, are an alternative to removing the inspection cover.

If the cover was already off, one would simply read the numbers that are stamped on the outside of the ring gear. While doing so, one could also verify that the carrier was, indeed, a limited slip, as worn clutches can mimic an open diff.

BTW: "Posi" is short for "Positraction", which was the name used only by Chevrolet, for marketing purposes.

Olds called theirs "Anti Spin".

Norm

[monzaz]

There are axle tube stamped numbers and letter suffixs on the passenger tube. BUT being that the cars is so old that is not the best way to tell. esspecial if someone repaired it over the years...never can tell.

BEST ALWAYS TO CRACK OPEN THAT COVER .Spin and wipe the ring gear outer edge diameter untill you find the ring gear and pinion gear tooth count like 16 41 or 15 41 divide the smaller pinion gear into the larger ring gear number and you will get your ratio. round up the 2nd digit after the decimal .

example ( 15 41 comes out to 2.7333333333) so no round up
example (13 40 comes to 3.07692307692 ) round up because of the 6 to 3.08...hope that helps. Jim

Oh and use it, as long as you get a good aftermarket Anti Spin (limited slip diff unit) or short LSD industry standard name for positraction/anti spin/ trac lok/suregrip etc. the rear will hold up very well. It is a 8.5 rear in 1971 1972 years. Good luck and if you need a quote on parts give us a shot at some prices. Jim

jdrace.com

[texasred]

i'd use the counting methods. My housing is a 2.78 open by the numbers but a previous owner put in 3.42 posi gears.

Cars are like a box of chocolates...

C.J.